1. Field of the Invention
The present invention generally relates to a semiconductor apparatus configured to function as a step-down type switching regulator, and especially relates to a semiconductor apparatus configured to function as a step-down type switching regulator wherein asynchronous rectification is carried out if a load current is small, and synchronous rectification is carried out when the load current is great.
2. Description of the Related Art
Rectification methods employed by step-down type switching regulators that use an inductor include a synchronous rectification method and an asynchronous rectification method. The step-down type switching regulators work in two modes, namely, a continuous operation mode and an intermittent mode. The continuous mode is where a great load current flows and a current continues flowing through the inductor. The intermittent mode is where the load current is small and the current may not always flow through the inductor.
FIG. 7 is a circuit diagram of an example of an output circuit of a conventional step-down type switching regulator using the synchronous rectification method.
According to the conventional step-down type switching regulator with the output circuit as shown in FIG. 7, efficiency is high in the continuous operation mode; however, efficiency is remarkably degraded in the intermittent mode, because a reverse current flows from a load (Vout) to ground potential through a transistor M102 for synchronous rectification.
When the load is light, switching between a switching transistor M101 and the transistor M102 for synchronous rectification is frequently carried out, which generates a switching loss. In order to mitigate the switching loss, switching to PFM control from PWM control is often performed.
Further, while the PFM control takes place, the output circuit is arranged to provide asynchronous rectification in order to prevent efficiency degradation by the reverse current. As shown in FIG. 8, a common practice is to use a diode D101 as a component for rectification in the output circuit of the step-down type switching regulator using the asynchronous rectification method. According to the circuit shown in FIG. 8, the reverse current can be prevented because a potential on the side of the load serves as a backward bias for the diode D101 even if the intermittent mode takes place. However, since the diode D101 has a relatively great forward voltage such as 0.6 V, power consumption by the diode D101 is great, and the efficiency cannot be remarkably improved.
In this connection, Patent Reference 1, for example, discloses a circuit of a switching regulator as shown in FIG. 9, whereby efficiency of the asynchronous rectification method is improved.
As shown in FIG. 9, the circuit includes a bipolar transistor Q101 for switching, which transistor is a PNP transistor. A driving signal is provided to the base of the bipolar transistor Q101, wherein the driving signal is provided by a PWM comparator that is not illustrated. If the driving signal is H (high level) and the bipolar transistor Q101 is turned off, a voltage V101 at an end of an inductor L101 falls to a negative voltage. The circuit further includes a comparator CMP101. A non-inverting input terminal of the comparator CMP101 is connected to ground potential. An inverting input terminal of the comparator CMP101 is provided with the voltage V101 at a connecting point of the bipolar transistor Q101 and an inductor L101. The comparator CMP101 has hysteresis.
If the voltage V101 is a negative voltage, an output of the comparator CMP101 becomes H (high level). The output of the comparator CMP101 is provided to the gate of a MOS transistor M102 for rectification and the MOS transistor M102 is turned on. Accordingly, no current flows into the inductor L101. If the voltage V101 becomes greater than ground potential, the output of the comparator CMP101 is L (low level), and turns off the MOS transistor M102 such that a reverse current from the load (Vout) is prevented. By using a device having an ON resistance that is smaller than a Schottky diode D101 as the MOS transistor M102 for rectification, the efficiency of the asynchronous rectification is raised.
[Patent Reference 1] JP 3402983